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In various electronic circuits,it is necessary to combine or separate AC and DC Voltages or Currents from eachother.This task is usually accomplished by use of capacitors.This so called coupling capacitors open circuits for DC Voltages and closed circuit for AC Signals.Hope this will you.
Power supply decoupling provides three benefits to a design: (1) improves
power supply integrity, (2) improves signal integrity, and (3) helps control EMI.
Generally, designs with good power supply and signal integrity have the fewest EMI
problems, and designs with good power supply integrity provide the best electrical
environment for signal transmission. Proper decoupling—the placement of capacitance
between power and the return (usually ground)—can make the difference
between a design working marginally and one that is reliable. It’s usually difficult to
retrofit decoupling capacitance into a marginal design to turn it into one that works
reliably. It’s always better to properly design in the power supply and return path
decoupling.
The EMI benefits are discussed in [12] and are not further discussed here other
than to observe that reducing the loop area traversed by signals is beneficial in
reducing emissions. As described in Chapter 4, this also improves signal integrity.
Power supply integrity has been extensively studied in the literature [13–18]. A
discussion of this work follows, with a focus on multilayer boards incorporating
power and ground planes. Double-sided boards that use wide traces to route power
to all of the elements are generally not used in high-speed design and so are not discussed
here.
I often see a couple caps placed in parallel on the board, each may be say, 10 nF or one is 10 nF and one is 0.1 uF. Why do this? Why not just add one 0.2 uF cap and get even more smoothing with fewer parts for manufacturing? I can see having multiple caps at opposite ends of the board, but it doesn't make sense to me to have two caps grounding one pin. What am I missing?
The reason that different value capacitors are used in by-passing or decoupling is to avoid the self resonance and lossy performance that one capacitor may exibit. A .1 uF capacitor may be a good low impedance bypass at 500 kHz, but usually not very effective at 800 MHz. By using two capacitors in parallel, like a 270 pF for the Higher frequencies, and a .1 uF for the lower frequencies, you will achieve better overall by-pass performance.
Coupling caps are usually used in circuits where only AC signals are passed through, for example, an RF amplifier has ac coupling caps at input and output, so that its dc biasing is isolated from the preceeding and subsequent circuits.
De-coupling caps, which are used to "short out" any ac component (ripple, switching noise, etc.) on the DC lines to keep DC voltages clean. Different types of caps have different frequency response, for examples, a high value cap such as 10uF tantulum caps has a much lower resonant frequency than a 100pF NPO ceramic type cap. When they are connected in parallel from DC line to ground, they "short out" a wide range of ac frequencies to keep the DC voltages clean.
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